Lithographic printing is carried out by feeding both water and oleophilic colored ink to the printing plate surface to allow the image areas to receive preferentially the colored ink and the non-image areas preferentially water and then transferring the ink deposited on image areas onto a substrate such as paper for example. Therefore, in order to obtain a print of good quality, it is necessary that oleophilicity of the image areas and hydrophilicity of the non-image areas are both strong enough so that when water and ink are applied, the image areas may receive sufficient amount of ink while the non-image areas may completely repel the ink. For this purpose, various methods for rendering the non-image areas insensitive to oil have been studied according to the type of printing plate materials and the printing methods.
That is, hitherto, as lithographic printing plate materials, there are presensitized plates (PS plates) with a support of metals such as aluminum, plate materials obtained by electrophotographic process (such as electro fax), so called master papers comprising a paper as substrate coated with a pigment together with a binder and photographic printing plate materials using silver salts as disclosed in Japanese Patent Kokoku No. 48-30562. Plate making methods and printing methods using solutions to render the non-image areas insensitive to oil and dampening solutions suitable for each of these materials have been studied and developed.
Lithographic printing plates which utilize transfer silver images obtained by silver complex diffusion transfer process as ink receptive areas are disclosed in Japanese Patent Kokoku No. 48-30562 and Papanese Patent Kokai Nos. 53-21602, 54-103104 and 56-9750 and are well known
The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York, (1972).
In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image-receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image") with respect to the black silver image obtained in the exposed areas of the photographic material.
A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellant ink-receptive areas on a water-receptive ink-repellant background. For example, typical lithographic printing plates are disclosed e.g. EP-A-423399 and EP-A-410500.
The DTR-image can be formed in the image-receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR-element) or in the image-receiving layer of a so-called single-support element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image-receiving layer in waterpermeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.
According to the preferred embodiment of the mono-sheet DTR-offset printing plate, disclosed in e.g. U.S. Pat. No. 4,722,535 and GB-P-1,241,661, a flexible support e.g. of paper or of an organic resin is provided in the order given with a silver halide emulsion layer and a layer containing physical development nuclei serving as the image-receiving layer. After information-wise exposure and development the imaged element is used as a printing plate without the removal of the now-useless emulsion layers.
Said type of mono-sheet DTR offset printing plates is not compatible with other types of printing plates with regard to dampening solutions. In order that said mono-sheet DTR offset printing plates show no ink acceptance in the non-printing areas (no staining), use should be made of special dampening solutions containing a seizable amount of a transparent pigment usually colloidal siliciumoxide as disclosed e.g. in U.S. Pat. Nos. 3,829,319, 4,238,279 and EP-A 304.662.
For economical reasons (less packaging material, less storage space, a smaller volume to transport) practically all dampening solutions, including the above mentioned dampening solutions are sold in a concentrated form and only diluted at the moment they are used on the printing press.
However, the above mentioned concentrated dampening solutions show a lack of shelf life due to the presence of this transparent pigment in said solutions having typically a pH between 3 and 6. Indeed, the lithographic active transparent pigment, mostly colloidal silica is not stable at said pH but is only stable at an alkaline pH of around 9. Colloidal silica which is stable at a pH below 7 is not lithographic active. In order to keep said dampening solutions homogeneous (which is a metastable situation for said solution) for some time, the presence of a water-soluble organic solvent is required.
The requirement for said dampening solutions of having a pH between 3 and 6 stems from the fact that otherwise said dampening solutions are not compatible with the usual lithographic inks. To maintain the required pH, said dampening solutions has to comprise a buffer because otherwise the pH will vary due to different causes e.g. the dilution with tap water, the contact with the usually alkaline printing paper etc.
As a buffer, citrate salts have been used. However, said dampening solutions comprising citrate salts form by storage a coarcevate and are not longer suitable for use. Also as a buffer, phosphate salts have been used. However storage of said dampening solutions containing phosphate salts results in floculation or even formation of a sediment of said transparent pigment. This results in a cloudy outlook of the undiluted as well as of the diluted dampening solution. Furthermore, this instability may cause a soiling of the dampening unit of the printing apparatus. Possibly, other parts of the press may also get soiled in due time. When a dampening circulation system is used, which delivers dampening solution to one or several presses, this system is also very prone to get soiled. This soiling of the dampening unit or the dampening circulation system results in the need for a more frequent cleaning of said units (a cumbersome and time-consuming operation). Apart from soiling said instability of said dampening solutions containing phosphate salts may even results in a possible staining in the non-image areas of the printed copies because less active transparent pigment is present in the dampening solution.